Hello everyone! Is there a difference how to start a row? When i was though to knit, I was told not to knit the first stich of the row, but I cant remember why or on what occasion. For example is there a difference when you knit a scarf or sleeve for the gardigan? Thank you.

It depends on what kind of edge you would like. Sometimes it's easier to seam if there is a stockinette edge (knit on one side, purl on the other) and I've seen recommendations to keep the edge in garter st (knit the edge st) for easy seaming.
For scarves, I like to slip the first st in the row and knit the last stitch in order to get a neat chain edge that looks very clean.
It's basically personal preference. Try different kinds of edges and see which you prefer.

If you have an exposed edge, like on a scarf, many people like to slip the first stitch every row to make a chained edge. My edges are looser and messy if I do that, so I just work the first stitch then do the next couple a little tighter and get neat even edges. You don't need to slip the edge if it will be seamed or have sts picked up in it as they won't show anyway.